The broad objective of the proposed research is the genetic dissection of a large number of complex and quantitative traits in the nematode worm and model organism C. elegans, with a focus on two classes of traits with relevance to human health: responses to pathogens and drugs. Success in understanding the genetic basis of phenotypic variation in a metazoan will provide critical guidance for the design of genotype-phenotype studies in humans and other organisms of medical, biological, and agricultural interest. The methods and resources developed will be broadly applicable to other phenotypes in C. elegans. The results will improve our understanding of the genes and pathways involved in susceptibility to pathogens, and in the mechanisms of action, resistance, and off-target effects of chemotherapeutics, anthelmintics, pesticides, and other compounds. Specifically, we will develop high-throughput quantitative phenotyping assays for these traits and apply them to genetic resources developed during the previous project period: a large set of high-resolution advanced intercross recombinant inbred lines from a cross between Bristol and Hawaii isolates, and a diverse collection of wild isolates extensively characterized for sequence variation. We will also apply these assays to new genetic resources that we will develop as part of the proposed research: we will build and genotype mapping populations from a maximally diverse subset of wild isolates. We will also develop new approaches for rapid identification of quantitative trait loci for any starting set of parent strains. We expect these efforts to produce:(i) a set of well-characterized diverse wild isolates and a broadly useful multiparent mapping population that will be shared with the C. elegans research community; (ii) new mapping methods applicable to C. elegans and other species; and (iii) a large set of loci for further investigation. We then propose to identify the genes and polymorphisms that underlie these loci, and to investigate the genetic architectures of the traits, including the population frequencies of the relevant alleles. We will confirm candidate genes and regions by using RNAi and transgenics to knock down or express genes in the appropriate strains. We will measure the frequencies of the identified alleles in the full diverse collection of wild isolates, and answer questions about rare vs. common alleles, additivity vs. dominance, and the role of genetic interactions. We expect to elucidate key principles of genetic architecture that will guide study design in C. elegans and other species. The pathways C. elegans uses to respond to biotic and abiotic stresses are conserved in humans and involved in a variety of diseases, including cancer and diabetes. Thus, we will leverage the power of the worm to better understand human biology.